Distances to globular clusters are determined either through fits of their color-magnitude diagrams (CMDs) with theoretical isochrones [1]}, [2]}, [3]}, [4]},
or by using variable stars that follow known relations between their periods and absolute luminosities like RR Lyrae stars [5]}, [6]}, Type II Cepheids [7]} or Mira type variables [8]}.
In order to avoid circularity, the absolute luminosities of these stars need to be determined independently from globular cluster distances by using for example theoretical models [9]} or Hipparcos or
Gaia parallaxes [10]}, [11]}, [12]}.
Accurate distances have also been obtained for globular clusters by using eclipsing binaries [13]}, [14]}, however the faintness and scarcity of suitable binaries
means that observations have so far been limited to a few globular clusters. Finally it is possible to determine distances by comparing the magnitudes of main-sequence stars with stars of similar metallicity in the solar
neighborhood, the so-called subdwarf method [15]}, [16]} or kinematically by comparing line-of-sight and proper motion velocity dispersion profiles in globular clusters
[17]}, [18]}, [19]}. The latter method has the advantage that the derived distance is not influenced by the reddening of the cluster.
